The present invention is for a method for producing a catalyst for use in the Fischer-Tropsch process, and the catalyst produced by the inventive method. The catalyst of the present invention comprises iron and at least one promoter. The catalyst is prepared via a method which comprises the preparation of a high purity iron precursor and which uses a nominal amount of water in the catalyst production. The catalyst particles prepared with the high purity iron precursor are essentially free of contaminants, and have essentially spherical particle shape and a relatively small particle size distribution range.
The Fischer-Tropsch synthesis involves the catalytic conversion of synthesis gas (a mixture of predominantly carbon monoxide and hydrogen) to a broad spectrum of saturated and unsaturated hydrocarbons ranging from methane to heavy wax. Oxygenates such as alcohols, ketones, aldehydes and carboxylic acids can also be synthesized by the Fischer-Tropsch synthesis. The first commercial Fischer-Tropsch catalysts were cobalt-based and were in use as early as 1935 in Germany. Early in the development of the Fischer-Trosch synthesis there was an interest in development of catalysts with metals less expensive than cobalt. Iron was an obvious choice; however, the commercial use of iron-based Fischer-Tropsch catalysts was not accomplished until the 1950's. Since that time iron-based Fischer-Tropsch catalysts have been successfully used in fixed-bed, fluidized-bed and slurry phase reactors on a commercial scale in South Africa by Sasol.
The activity and selectivity of iron-based Fischer-Tropsch catalysts are greatly improved by the addition of small amounts of promoters. The classic iron-based Fischer-Tropsch catalyst is promoted with copper and a group I metal, such as sodium, potassium, rubidium, cesium or a combination thereof Iron-based Fischer-Tropsch catalysts are active only when they have been reduced with hydrogen, carbon monoxide or synthesis gas. Copper has been found to significantly lower the reduction temperature of iron oxide and thus prevent sintering of the catalyst. Promotion with a Group I metal, such as potassium, lowers the acidity of the iron oxide and thereby decreases the selectivity to undesirable methane and increases alkene and wax selectivity. Group II metals may also be used; however, Group I metals are more effective promoters. Binders such as SiO2 and Al2O3, can also be used to increase the structural integrity and life of iron-based catalysts; however, these generally are acidic and will result in an increase in methane selectivity.
There have been several methods used for the preparation of iron-based Fischer-Tropsch catalysts. The earliest catalysts, prepared by Fischer, were iron turnings treated with alkali. At high pressure, the liquid product was rich in oxygenated compounds, and at lower pressures hydrocarbons were produced. However, the iron-based catalysts prepared by this method deactivated rapidly.
The most common method of preparation of iron-based Fischer-Tropsch catalysts is precipitation. Typically a solution of an iron salt, such as ferric nitrate, is treated with a base, such as aqueous ammonia or sodium carbonate. The resulting iron oxyhydroxide precipitate is washed and filtered repeatedly to remove salts—ammonium nitrate or sodium nitrate—formed during the precipitation process. The washed filter cake is then dried and calcined. Promotion of the precipitated iron catalyst with copper and a Group I metal can be done at any time, before or after the drying and calcination steps. The final catalyst is usually composed of high surface area corundum phase iron oxide (α-Fe2O3 or hematite).
Other types of iron based catalysts include, fused iron, supported iron and sintered iron. Fused iron catalysts are prepared by melting iron ore and one or more promoter such as SiO2, Al2O3, CaO, MgO and K2O. The resulting catalyst is usually composed predominantly of magnetite (Fe3O4) and has very low surface area. Active fused iron catalysts can only be achieved by reduction of the oxide to metalic iron with hydrogen. The reduced catalyst can have surface area up to about 10 to 15 m2/g. Fused iron catalysts are characterized by high structural integrity and as such are well suited for fluid bed operations (Sasol); however, the relatively low surface area results in a Fischer-Tropsch catalyst with inferior activity as compared to typical precipitated iron catalysts. Supported iron catalysts are usually prepared by impregnating a solution of an iron salt onto a refractory metal oxide such as Al2O3, SiO2, TiO2 or ZrO2. The impregnation can be carried out by incipient wetness techniques or by excess wetting followed by vacuum drying. Supported iron catalysts can have Fischer-Tropsch activity similar to precipitated iron catalysts on an iron mass basis; however, they are typically inferior on a catalyst volume basis. Supported iron catalysts inevitably suffer from the acidity of the metal oxide supports which increases the selectivity of undesirable methane.
Precipitated iron catalysts are generally regarded as superior Fischer-Tropsch catalysts to the other types of iron catalysts described herein. The major disadvantages of the manufacture of precipitated iron catalysts include high cost, the method is labor intensive, and the by-products are deleterious to the environment. Iron nitrate is the preferred iron source of precipitated iron catalysts because chloride and sulfur contamination from iron chloride or iron sulfate would have a deleterious affect on the activity of the resulting F-T catalyst. Iron nitrate is manufactured by the digestion of iron metal in nitric acid which produces nitrogen oxides that must be recovered by a scrubbing process. This necessary scrubbing step adds additional cost to the process. Further, the precipitation method tends to result in the formation of very viscous and gelatinous iron hydroxide or iron oxyhydrate precursor. This viscous precursor can be very difficult to form into spherical and attrition-resistant catalyst for fluid bed applications.
A process to produce iron-based Fischer-Tropsch catalysts that reduces or eliminates the washing and filtration steps and has minimal emissions to the environment would be favorable. A logical process from a commercial viewpoint would be to promote, form, dry and calcine a commercially available iron oxide that has high purity and high surface area. Commercial iron oxides are readily available; however, they are usually prepared by treatment of steel with hydrochloric acid or sulfuric acid. These iron oxides contain significant amounts of impurities including chloride and sulfur which makes them unusable as raw materials for Fischer-Tropsch catalysts. As is known in the art, the impurities of the commercial iron oxides (red or yellow iron oxides) can be reduced to very low level by the pickling process under very high temperatures. However, because of the extreme conditions of the pickling process, the surface area of the iron oxide is generally less than 10 m2/g making the iron oxide unsuitable for catalyst applications.